1. Field of the Invention
This invention relates to the operation of a cooling fluid system for a production plant wherein a gravity-drained cooling fluid return header system is employed to feed a cooling tower complex.
More particularly, this invention relates to the operation of a gravity drained cooling fluid return system for a multi-cell cooling tower complex which return system provides a uniform flow of hot cooling fluid to each cooling cell of that complex.
This invention is especially applicable to cooling fluid return systems that employ a single return header, which header feeds multiple return sub-headers wherein each sub-header feeds returned hot cooling fluid to an individual cooling cell in a multi-cell cooling tower complex.
2. Description of the Prior Art
For sake of clarity and brevity, this invention will be described in relation to a polyethylene production plant that uses an exothermic process for forming polyethylene in water cooled reactors. Accordingly, this plant uses liquid water (hereafter “water”) as a cooling fluid for the reactor(s) in the plant, and a cooling tower complex consisting of five individual cooling cells that operate synchronously. However, this invention is not limited to such a production plant, cooling tower system, or water as a cooling fluid.
In a cooling tower system that employs a plurality of cooling cells that are each fed by a gravity-drained sub-header that is in fluid communication with a closed end (blind), common cooling water return header pipe (conduit), the challenge is to obtain uniform water flow through each sub-header take-off pipe so that the heated return water is evenly distributed among the various individual cooling cells.
Cooling of the polymer producing reactors in the plant can be a limiting factor for the production rate of those reactors, particularly in the warmer months of the year. The goal is to supply cooling water to the plant that is consistently as close as possible to the ambient temperature of the plant, hence the drive for even distribution of hot cooling water to the cooling cells.
With even distribution of returned, hot cooling water between all the cooling cells of the cooling tower complex, maximum ambient cooling of the return water is achieved, which, in turn, helps maximize the polymer production rate of the plant as a whole.
This even distribution of returned cooling water to all the cells in the cooling tower complex is particularly challenging in a gravity-fed return system when the take-off points for the sub-headers from the common header are at different locations along the vertical height of that header. This invention meets that challenge.
As will be seen in greater detail hereinafter, balancing gravity-fed water return flows between multiple sub-headers from a common header is readily achieved by this invention as a matter of routine in a timely manner.
Heretofore, this return water balancing act was attempted by employing a manually operated butterfly valve mounted in each sub-header. Each such valve was manually opened or closed in an effort to get uniform flow through each sub-header. Approximate uniform water flow through each sub-header and cooling cell was attempted to be achieved by simple visual observation by the person operating the valves of the volume of water falling through each of the cells, the operator manually opening and closing individual valves until approximately even volumes of water were observed by the operator to be flowing through each of the cells.
In reality, this prior art process, because of the hysteresis effect of flowing water as described in greater detail hereinafter, this goal was impossible to achieve in a reasonable period of time, and extremely difficult to achieve even if time was of no consequence, which is never the case in a commercial production plant. This was so even if the return water was taken off from the common header at the same height along that header. When the return water was taken off from different locations across the height of the common header, the problem of obtaining even flow of return water to all sub-headers was made immeasurably more difficult because it was possible, even probable, that all or a major portion of the return water (see FIG. 4 hereinafter) would be removed from the header by way of less than all of the sub-headers, thereby starving at least one sub-header and its associated cooling cell of return water flow altogether. This situation puts more return water to be cooled through less than all the cooling cells available in the complex, and prevents the cooling tower complex from achieving its goal of cooling the return water to as close as possible to the ambient temperature surrounding that complex.
One method for solving the even distribution of return water through the sub-headers could arguably be measuring the actual volume of water flowing in each sub-header, but this would entail the use of a complicated system of fluid flow measuring equipment that would be expensive to install and maintain. This invention employs less expensive equipment involving easier to detect liquid levels, as opposed to measuring actual liquid flow volumes, in each sub-header, and then using this liquid level data as a measure of uniform water flow in each sub-header and its associated cooling cell.